RFC 1918 Addresses

RFC 1918 addresses are IP addresses reserved for private use within internal networks. These addresses are not routable on the public internet, meaning they are used exclusively within private networks. The primary purpose of these addresses is to alleviate the shortage of IPv4 addresses and to enhance network security by isolating internal network traffic from the public internet.
 
RFC 1918 specifies three ranges of IPv4 addresses for private use:
 

Class A Private:
10.0.0.0 to 10.255.255.255 (10.0.0.0/8)

Class B Private:
172.16.0.0 to 172.31.255.255 (172.16.0.0/12)

Class C Private
192.168.0.0 to 192.168.255.255 (192.168.0.0/16)

These address ranges are commonly used in home, office, and enterprise networks. Devices within these networks can communicate with each other using these private IP addresses, but require a Network Address Translation (NAT) device to communicate with external networks or the internet.